This invention concerns the handling of plastic sheets in preparation for thermoforming of the sheets into molded articles. In the well known process of thermoforming, plastic sheets are preheated in an oven prior to being shaped by the use of forming molds and fluid pressure.
In one known form of the process, individual rectangular sheets are each clamped in a rectangular clamping frame to be transferred into an oven and thence into a forming station.
In published U.S. Application US 2002/0017745 A1, a slow preheating of the sheets is described with a second rapid heating in a radiant heating station just prior to being formed in a forming station.
Such preheating has been done in a convection oven with pairs of sheets slowly moved through the oven on an indexing vertical conveyor including paddles on which the sheets rest. The pairs of preheated sheets are conveyed out of the convection oven to be clamped into a clamping frame for further processing in a radiant heating station where final heating is done, and thereafter to the forming station.
The preheated sheets are moved into respective clamping frames where clamps grip the edges of the sheets to secure the same in their respective frame.
Problems have been encountered in achieving proper clamping due to bowing and curling of the sheets due to the prior preheating step.
In addition, extensive handling of the sheets are involved in feeding the sheets into the convection oven, conveying the same through the oven, and transferring the preheated sheets out of the oven. Such extensive handling sometimes results in the sheets being skewed when moved into the clamping frame to be slightly misaligned with the frame.
This misalignment can interfere with proper clamping of the sheets in their respective clamping frames.
While sheet squaring devices have heretofore been developed to correct this problem, a sheet flattening capability has not been provided in such devices.
It is the object of the present invention to provide a method and apparatus for providing sheet squaring and flattening insuring proper clamping of preheated sheets within the clamping frames of a thermoformer machine.